1. Field of the Invention
The present invention relates generally to the field of centrifugal compressors. More specifically, the present invention concerns a centrifugal compressor with a rotatable case insert for minimizing the risk of crashing caused by contact between the impeller and case wall.
2. Discussion of Prior Art
Centrifugal compressors are commonly employed in applications where fluid compression requirements involve high volumetric flow rates. Examples of such applications include liquid pumps and forced air induction systems using superchargers or turbochargers. Centrifugal compressors typically achieve high flow rates with vaned impellers that turn at extremely high rates of angular velocity, as high as 30,000 to 70,000 rpm. These high rotational speeds quickly expose wear and failure modes of the compressor.
For example, an impeller generally rotates within a chamber of a stationary compressor case. The vanes of the impeller rotate next to an annular wall formed as part of the case. Clearances between the impeller and the wall are designed to be tight in order to maximize compressor efficiency. One risk of this necessary arrangement is the possibility of contact between the impeller and case, which can cause the impeller and impeller bearings to experience violent shock loading and, in some instances, catastrophic compressor crashing or failure. It has been determined that such contact is sometimes attributable to localized superheating of the impeller and wall occurring along isolated areas where the clearance becomes too small. Superheating is also a function of the relative difference in angular velocity between the wall and impeller, referred to as the velocity variance. As superheating occurs, material from the impeller has a tendency to build up on the wall which effectively reduces the clearance. The superheating and associated material build-up also tend to rob the impeller of energy by reducing its speed. If the build-up of material continues to grow and is not eliminated, the impeller and wall can contact each other. In any case, there is a need for an improved centrifugal compressor that minimizes the risk of damage, poor performance, and catastrophic failure caused by contact between the impeller and case or superheating of the impeller and case.